Source file src/pkg/crypto/subtle/constant_time.go
1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Package subtle implements functions that are often useful in cryptographic 6 // code but require careful thought to use correctly. 7 package subtle 8 9 // ConstantTimeCompare returns 1 iff the two equal length slices, x 10 // and y, have equal contents. The time taken is a function of the length of 11 // the slices and is independent of the contents. 12 func ConstantTimeCompare(x, y []byte) int { 13 var v byte 14 15 for i := 0; i < len(x); i++ { 16 v |= x[i] ^ y[i] 17 } 18 19 return ConstantTimeByteEq(v, 0) 20 } 21 22 // ConstantTimeSelect returns x if v is 1 and y if v is 0. 23 // Its behavior is undefined if v takes any other value. 24 func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y } 25 26 // ConstantTimeByteEq returns 1 if x == y and 0 otherwise. 27 func ConstantTimeByteEq(x, y uint8) int { 28 z := ^(x ^ y) 29 z &= z >> 4 30 z &= z >> 2 31 z &= z >> 1 32 33 return int(z) 34 } 35 36 // ConstantTimeEq returns 1 if x == y and 0 otherwise. 37 func ConstantTimeEq(x, y int32) int { 38 z := ^(x ^ y) 39 z &= z >> 16 40 z &= z >> 8 41 z &= z >> 4 42 z &= z >> 2 43 z &= z >> 1 44 45 return int(z & 1) 46 } 47 48 // ConstantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged. 49 // Its behavior is undefined if v takes any other value. 50 func ConstantTimeCopy(v int, x, y []byte) { 51 xmask := byte(v - 1) 52 ymask := byte(^(v - 1)) 53 for i := 0; i < len(x); i++ { 54 x[i] = x[i]&xmask | y[i]&ymask 55 } 56 return 57 }